Lead is a continuing health and environmental hazard and its neurotoxic effects have been well documented. However, the mechanism for the entry of Pb2+ into the cell is not well understood. The novel instrumentation, the non-invasive Ca2+-selective referencing electrode (1,2), was used to test the hypothesis that Pb2+ can displace intracellular Ca2+ and measure any subsequent transmembrane ion movement. Our preliminary results showed that Pb2+ stimulates Ca2+ flux in a concentration-dependent manner. Furthermore, this Pb2+ -induced flux was abolished by the addition of verapamil (VPML), a known Ca2+ channel blocker. This leads us to propose that Pb2+ is entering the cells through Ca2+ channels. Studies have verified the involvement of Ca2+ channels in both vertebrate and invertebrate cell types (3,4,5). The sustained Ca2+ signals seen with Pb2+ and thapsigargin (TPGN) (an inhibitor of the endoplasmic reticulum Ca2+ pump) might indicate the enhanced uptake of Pb2+ an d we hypot hesize, the subsequent sequestration of Pb2+ inside intracellular stores. It has been shown that thapsigargin promotes Ca2+ release from intracellular stores thereby increasing cytosolic Ca2+ (6). Moreover, when it depleted intracellular Ca2+ stores, it stimulated uptake rates of Pb2+ into the cell (5). Pb2+-induced Ca2+ flux across the membrane of bag cell neurons was detected using the non-invasive self-referencing Ca2+ electrode. Addition of Pb2+ in 50?M increments (50-300?M concentration) yielded a corresponding increase in Ca2+ signals. Background voltage readings were relatively stable and unaffected with each introduction of the toxicant. To further determine if Pb2+ displaces Ca2+, verapamil, a Ca2+ channel blocker, was used as a pharmacological probe. Ca2+ patterns changed with the addition of Pb2+ and VPML. It not only demonstrated that Pb2+ enhances extracellular Ca2+ flux, but that VPML effectively shuts down Pb2+-induced Ca2+ flux. Despite the offset seen in the background voltage reading as VPML was added, the electrode retained its Nernstian quality and its response time stayed the same. On the other hand, the extracellular flux pattern seen after Pb2+ and TPGN were added to the cell medium was not as expected but interesting nonetheless. The expected marked increase in Ca2+ flux due to the combined action of the two substances were not observed. Instead a sustained signal was present for at least an hour of recording. References 1. Smith, P.J.S., Sanger, R.H. & Jaffe, L.F. 1994. In: Methods Cell Biol. 40: 115-134. 2. Smith, P.J.S. 1995. Nature 378: 645-646. 3. Audesirk, G. & Audersirk, T. 1993. Neurotoxicol. 14: 259. 4. Busselberg, D., Evans, ML, et al 1991. J. Neurophysiol. 65: 786. 5. Kerper, L.E. & Hinkle, P.M. 1997. J. Biol. Chem. 272:8346. 6. Knox, R.J., Kao, L..S., Jonas, E., Smith, P.J.S. & Kaczmarek, L.K. 1996. J. Physiol. 494: 627 639